bgpd: Install multipath routes with weights

bgpd: Install multipath routes with weights

Perform weighted ECMP if the multipaths have link bandwidth. This involves
assigning weights to each of the next hops associated with the prefix based
on the link bandwidth of the corresponding path as a factor of the total
(cumulative) link bandwidth for the prefix. The weight values used are
between 1 and 100. Weights are assigned only if all paths in the multipath
have link bandwidth, otherwise any bandwidths are ignored and regular
ECMP is performed. This is as recommended in
https://tools.ietf.org/html/draft-ietf-idr-link-bandwidth

A subsequent commit will implement additional (user-configurable) behaviors.

Signed-off-by: Vivek Venkatraman <vivek@cumulusnetworks.com>
Reviewed-by:   Donald Sharp <sharpd@cumulusnetworks.com>
Reviewed-by:   Don Slice <dslice@cumulusnetworks.com>